LM87CIMTX/NOPB National Semiconductor, LM87CIMTX/NOPB Datasheet - Page 15

• Manufacturer Part Number: LM87CIMTX/NOPB
• Description: IC MONITOR SYS HARDWARE 24TSSOP
• Manufacturer: National Semiconductor
• Series: PowerWise®r

Specifications of LM87CIMTX/NOPB

• Function: Hardware Monitor
• Topology: ADC (Sigma Delta), Comparator, Fan Speed Control, Register Bank
• Sensor Type: External & Internal
• Sensing Temperature: -40°C ~ 125°C, External Sensor
• Output Type: I²C™/SMBus™
• Output Alarm: No
• Output Fan: Yes
• Voltage - Supply: 2.8 V ~ 3.8 V
• Operating Temperature: -40°C ~ 125°C
• Mounting Type: Surface Mount
• Package / Case: 24-TSSOP
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Other names: *LM87CIMTX; *LM87CIMTX/NOPB; LM87CIMTX

The resistors were selected by setting R2 = 20 kΩ and then
calculating R1 using the following equation, ( V
mum negative input voltage, V
The maximum R1 can be is restricted by the DC input current
of an AIN input.
Inputs with internal resistor dividers (+2.5Vin, +3.3Vin or
+5Vin, +12Vin) can have voltage applied that exceeds the
power supply up to: 3.6 V for +2.5Vin, 4.6 V for +3.3Vin, 6.8
V for +5Vin, and 15 V for +12Vin. The AIN inputs have a par-
asitic diode to the positive supply, so care should be taken not
to forward bias this diode. All analog inputs have internal
diodes that clamp the input voltage when going below ground
thus limiting the negative analog input voltage range to −50
mV. Violating the analog input voltage range of any analog
input has no detrimental effect on the other analog inputs.
External resistors should be included to limit input currents to
the values given in the ABSOLUTE MAXIMUM RATINGS for
Input Current At Any Pin whenever exceeding the analog in-
put voltage range, even on an un-powered LM87. Inputs with
external attenuator networks will usually meet these require-
ments. If it is possible for inputs without attenuators (such as
AIN1 and AIN2) to be turned on while LM87 is powered off,
additional resistors of about 10 kΩ should be added in series
with the inputs to limit the input current.
4.1 Analog Input Interrupts
A WATCHDOG window comparison on the analog inputs can
activate the INT# interrupt output. A converted input voltage
that is above its respective HIGH limit or less than or equal to
its LOW limit will cause a flag to be set in its Interrupt Status
Register. This flag will activate the INT# output when its mask
bit is set low. Mask bits are found in the Interrupt Mask Reg-
isters. The Interrupt system is described in much greater
detail in Section 9.0.
5.0 LAYOUT AND GROUNDING
A separate, low-impedance ground plane for analog ground,
which provides a ground point for the GND pin, voltage di-
viders and other analog components, will provide best per-
formance, but is not mandatory. Analog components such as
voltage dividers should be located physically as close as pos-
sible to the LM87.
The power supply bypass, the parallel combination of 10 μF
(electrolytic or tantalum) and 0.1 μF (ceramic) bypass capac-
itors connected between pin 9 and ground, should also be
located as close as possible to the LM87.
R1 = [(1.25V − V
(a) Fan with Tach Pull-Up to +5V
S
) ÷ (V
+
is the positive pullup voltage):
+
− 1.25V)] × 20 kΩ
S
is the maxi-
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15
(b) Fan with Tach Pull-Up to +12V, or Totem-Pole Output and
6.0 FAN INPUTS
The FAN1 and FAN2 inputs accept signals from fans
equipped with tachometer outputs. These are logic-level in-
puts with an approximate threshold of V
ing in the LM87 accommodates the slow rise and fall times
typical of fan tachometer outputs. The maximum input signal
range is 0 to V
fan outputs which exceed 0 to V
diode clamping must be included to keep inputs within an ac-
ceptable range, as shown in
it does not develop excessive voltage due to input leakage.
R1 is selected based on R2 to provide a minimum input of 2 V
and a maximum of V
provide the maximum possible input up to V
immunity. Alternatively, use a shunt reference or zener diode
to clamp the input level.
If fans can be powered while the power to the LM87 is off, the
LM87 inputs will provide diode clamping. Limit input current
to the Input Current at Any Pin specification shown in the AB-
SOLUTE MAXIMUM RATINGS section. In most cases, open
collector outputs with pull-up resistors inherently limit this cur-
rent. If this maximum current could be exceeded, either a
larger pull up resistor should be used or resistors connected
in series with the fan inputs.
The Fan Inputs gate an internal 22.5 kHz oscillator for one
period of the Fan signal into an 8-bit counter (maximum count
= 255). The default divisor, located in the VID/Fan Divisor
Register, is set to 2 (choices are 1, 2, 4, and 8) providing a
nominal count of 153 for a 4400 rpm fan with two pulses per
revolution. Typical practice is to consider 70% of normal RPM
a fan failure, at which point the count will be 219.
Determine the fan count according to:
Note that Fan 1 and Fan 2 Divisors are programmable via the
VID/Fan Divisor Register.
Fan tachometer outputs that provide one pulse per revolution
should use a divisor setting twice that of outputs that provide
two pulses per revolution. For example, a 4400 RPM fan that
provides one pulse per revolution should have the divisor set
to 4 such that the nominal counter output is 153.
+
. In the event these inputs are supplied from
Resistor Attenuator
+
. R1 should be as low as possible to
Figure
+
, either resistive division or
7. R2 is selected so that
+
/2. Signal condition-
+
for best noise
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10099513